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A thermophilic two-stage fermentation process using 10% total solids (TS) food waste was tested at varying organic loading rates (OLRs). The system was configured to produce H(2) and CH(4) in conjugation with the chemical oxygen demand (COD), nitrogen removal, and adjustment of the pH by returning sludge as an alkali buffer from the sludge storage tank for denitrification. The pH in the H(2) fermentation reactor was maintained in the range of 5.4-5.7 using sludge recirculation (Q(r)/Q(i) ratio 1). The average H(2) (11.1l-H(2) l(-1)-fed d(-1)) and CH(4) (47.4l-CH(4) l(-1)-fed d(-1)) production rates were achieved at OLRs of 39 (H(2) fermentation reactor) and 4.16 gCOD l(-1)d(-1) (CH(4) fermentation reactor), respectively. These results suggest that long-term stability of the continuous two-stage process can be successfully achieved by recirculation of high-alkalinity sludge of 6.7-7.5 g l(-1) as CaCO(3), without any added external chemical buffer.

Abstract: The influence of solids retention time (SRT) on continuous H2 production in a submerged membrane bioreactor (MBR) was investigated using mixed mesophilic microflora. The bioreactor was continuously operated at the four SRTs of 2, 4, 12.5 and 90 d on a glucose medium under the hydraulic retention time (HRT) of 9h and the mesophilic condition of 35°C±0.5. Stable biogas production with H2 content of 50.8%–60% was achieved at SRTs ranging from 2 to 12.5 d. No methane gas was observed in monitoring the experimental conditions. The H2 production increased from 17.62 to 26.1l-H2/d when the SRT increased from 2 to 12.5 d, but decreased to 9.1l-H2/d at the 90 d SRT. The best H2 yield, 1.19mol-H2/mol-glucose, was observed at the SRT of 2 d and the highest H2 production rate, 5.8l-H2/l/d, was obtained at the SRT of 12.5 d. Stable H2 production was achieved by maintaining the SRT in the range of 2 - 12.5 d, regardless of the fermentative pathway related to higher lactate production. The decrease in H2 yield was observed at long SRTs due to the low volatile suspended solids/total suspended solids (VSS/TSS) as well as the high extracellular polymeric substances (EPS) concentrations. These results suggest that the SRT is the key factor enabling sustainable H2 fermentation in MBR, and that an SLR value of around 1.6kg-DOC/kg-VSS/d might be the specific condition for achieving optimum H2 production. [Copyright &y& Elsevier]

Abstract: A thermophilic two-stage fermentation process using 10% total solids (TS) food waste was tested at varying organic loading rates (OLRs). The system was configured to produce H2 and CH4 in conjugation with the chemical oxygen demand (COD), nitrogen removal, and adjustment of the pH by returning sludge as an alkali buffer from the sludge storage tank for denitrification. The pH in the H2 fermentation reactor was maintained in the range of 5.4–5.7 using sludge recirculation (Q r/Q i ratio 1). The average H2 (11.1l-H2 l−1-fed d−1) and CH4 (47.4l-CH4 l−1-fed d−1) production rates were achieved at OLRs of 39 (H2 fermentation reactor) and 4.16gCOD l−1 d−1 (CH4 fermentation reactor), respectively. These results suggest that long-term stability of the continuous two-stage process can be successfully achieved by recirculation of high-alkalinity sludge of 6.7–7.5gl−1 as CaCO3, without any added external chemical buffer. [Copyright &y& Elsevier]

[Display omitted] • Microbial electrosynthesis cells (MESs) were assessed for AD biogas upgrading. • Electrode-attached cells play a key role in converting CO 2 to CH 4 in an MES. • The MES produced 10.55L CH 4 /m2 cat /day with a final content of 96% • 8.8L CH 4 /m2 cat /day and 95% was achieved using real AD biogas. • The energy efficiency of CH 4 conversion was compared in a lab and bench-scale MES. Upgrading biogas from anaerobic digestion (AD) has been highlighted as an alternative renewable energy source to replace geopolitically limited natural gas. The CO 2 content of AD effluent is more than 40%, making it necessary to separate or increase the CH 4 content to 95%. This study examined microbial electrosynthesis (MES) to convert CO 2 directly to CH 4 by a cathode electrode-attached cell. The MES with a −1.0 V (versus Ag/AgCl) applied cathodic potential exhibited a maximum methane production rate of 10.55 L CH 4 /m2 cat/day and achieved a 96% final CH 4 content. Applying real biogas from a field AD plant resulted in a comparable production rate of 8.8L CH 4 / m2 cat/ day with 95% CH 4. The scaled-up bench MES reactor (total volume of 6L) was evaluated, and the energy efficiencies of the laboratory- and bench-scale MES were compared. The next-generation sequencing (NGS) revealed most methanogens (e.g., Methanobacterium, Methanothrix, and Methanobrevibacter) to be associated with the cathode surface rather than suspension. Cyclic voltammetry and field emission scanning electron microscopy showed that the electrode-associated cell predominantly controls the performance of the MES system. These results suggest that electrode-attached cells play a major role in the biogas upgrading of CO 2 to CH 4 in the MES system. [ABSTRACT FROM AUTHOR]

Abstract: Bioenergy produced from co-digestion of sewage sludge (SS) and rice straw (RS) as raw materials, without pretreatment and additional nutrients, was compared for the one-stage system for producing methane (CH4) and the two-stage system for combined production of hydrogen (H2) and CH4 in batch experiments under thermophilic conditions. In the first stage H2 fermentation process using untreated RS with raw SS, we obtained a high H2 yield (21 ml/g-VS) and stable H2 content (60.9%). Direct utilization of post-H2 fermentation residues readily produced biogas, and significantly enhanced the CH4 yield (266 ml/g-VS) with stable CH4 content (75–80%) during the second stage CH4 fermentation process. Overall, volatile solids removal (60.4%) and total bioenergy yield (8804 J/g-VS) for the two-stage system were 37.9% and 59.6% higher, respectively, than the one-stage system. The efficient production of bioenergy is believed to be due to a synergistically improved second stage process exploiting the well-digested post-H2 generation residues over the one-stage system. [Copyright &y& Elsevier]